It symbolizes the end of Boo Radley's attempts to communicate with the children. He communicates with them through the gifts he leaves for them.
If you sit down, and stare down at your phone, or lay your head on a table for too long your neck gets strained. If you are hunched for too long your back starts to strain and you will feel pain on your back.
The answer you are looking for here is C.
Characterization and events in the story may point to a story’s implied theme.
